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I’m Dreaming of a Salty Christmas

Every winter, when the road salt hits the streets, I can’t help but wonder about its environmental consequences. In the US alone, approximately 35 billion pounds of salt are used each year to melt ice and snow from streets and sidewalks. Salt acts by lowering the freezing point of water and functions optimally at temperatures above -10 Celsius (15 F). Lower than that, salt has little effect. Salt is used by many cities in the northern US, while many Canadian cities instead use sand, with the goal of improving traction rather than melting snow and ice.

Although we generally think of road salt as having the same composition as table salt (NaCl), other salts including MgCl can also be used

A recent study by the US Geological Survey shows a clear increasing trend in salt concentrations of rivers in Wisconsin, Ohio, and Illinois over several decades. In Wisconsin in particular, salt levels in the early spring were recorded as 10 to 15 times higher than the federal limit. Similar studies have shown that concentration trends in lakes across the northern US have closely followed the annual growth of road salt purchases, while sites sampled in 13 cities also exceeded EPA limits.

Some contaminants released into the environment by humans are harmful even in small quantities. An example is benzene, a hydrocarbon byproduct released through tobacco smoke, car exhaust, and coal emissions. Benzene has been linked to cancer, blood disorders, and developmental defects in infants. In the US, benzene is tightly regulated by the EPA, with a maximum allowed concentration of 5 parts per billion (ppb) in water (one molecule of benzene per one billion molecules of water).

Molecular structure of benzene, with hydrogen shown in white and carbon in black

On the other hand, chemicals such as salt have much higher acceptable concentrations. The limit for salt in water is 230 ppm, set not by human health considerations but by the health of aquatic species and ecosystems. For humans, water will taste too salty to drink well before it poses a health concern. Excess salt in water bodies can threaten fish and aquatic ecosystems, while on land it can harm vegetation and reduce soil fertility.

The question is not one of stopping salt use but of preventing overuse. When optimized, road salt use significantly reduces the risk of car accidents in icy conditions. Overuse, however, presents both economic and environmental costs. A close parallel can be drawn between road salt and fertilizer use. Fertilizer containing phosphorus and nitrogen is widely used to improve crop yields. However, excess phosphorus and nitrogen drains into rivers and lakes, where it can have severe consequences. Lake Winnipeg, shown below, is a dramatic example where overabundance of nutrients have allowed blooms of algae to develop on the surface, killing other organisms and producing toxins dangerous to humans. Management of both road salt and fertilizer requires education of users as to how much of the applied chemical is ‘necessary’ and how much is ‘excess’, as well as incentives or penalties to promote responsible usage.

Algal blooms on Lake Winnipeg show up in bright green on this satellite image

Be warned: you may have just sustained a lethal dose of mostly harmlessscience.

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